Difference between revisions of "Gzip"

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== File format ==
 
The gzip file (.gz) format consists of:
 
The gzip file (.gz) format consists of:
* a 10-byte header, containing a magic number (1f 8b), a version number and a timestamp
+
* a file header
 
* optional extra headers, such as the original file name,
 
* optional extra headers, such as the original file name,
 
* a body, containing a DEFLATE-compressed payload
 
* a body, containing a DEFLATE-compressed payload
 
* an 8-byte footer, containing a CRC-32 checksum and the length of the original uncompressed data.
 
* an 8-byte footer, containing a CRC-32 checksum and the length of the original uncompressed data.
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 +
=== File header ===
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The file header is 10 bytes in size and contains:
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{| class="wikitable"
 +
! align="left"| Offset
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! Size
 +
! Value
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! Description
 +
|-
 +
| 0
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| 2
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| 0x1f 0x8b
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| Signature (or identification byte 1 and 2)
 +
|-
 +
| 2
 +
| 1
 +
|
 +
| Compression Method
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|-
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| 3
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| 1
 +
|
 +
| Flags
 +
|-
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| 4
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| 4
 +
|
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| Last modification time <br> Contains a POSIX timestamp.
 +
|-
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| 8
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| 1
 +
|
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| Extra flags
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|-
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| 9
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| 1
 +
|
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| Operating system <br> Value that indicates on which operating system the gzip file was created.
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|}
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==== Extra flags ====
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If compression method is 8 the following extra flags can be defined:
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* 0x02 - compressor used maximum compression, slowest algorithm
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* 0x04 - compressor used fastest algorithm
  
 
== External Links ==
 
== External Links ==

Revision as of 01:31, 28 November 2013

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File format

The gzip file (.gz) format consists of:

  • a file header
  • optional extra headers, such as the original file name,
  • a body, containing a DEFLATE-compressed payload
  • an 8-byte footer, containing a CRC-32 checksum and the length of the original uncompressed data.

File header

The file header is 10 bytes in size and contains:

Offset Size Value Description
0 2 0x1f 0x8b Signature (or identification byte 1 and 2)
2 1 Compression Method
3 1 Flags
4 4 Last modification time
Contains a POSIX timestamp.
8 1 Extra flags
9 1 Operating system
Value that indicates on which operating system the gzip file was created.

Extra flags

If compression method is 8 the following extra flags can be defined:

  • 0x02 - compressor used maximum compression, slowest algorithm
  • 0x04 - compressor used fastest algorithm

External Links